A few hundred years ago (actually over 500 years ago), people didn't have enough ways to gather evidence to prove or disprove it, so they went with their own eyes: the horizon looked flat, ergo it must have been flat! Your eyes though, have been telling you and billions of people over thousands of years that there are stars on the sky.

And (I'm remembering now Rumsfeld's quote about the "unknown unknowns"), it's true that something can't be discarded just because nobody else agrees with it. Nobody thought about evolution before Darwin's age. But, the thing is, you can't accept something as a fact unless you have evidence to back it up. It can't be disproven, but it can't be proven either, and something is not true unless it's proven. Ergo, as it's not proven, it's not true. You don't need to disprove something that has been never proved before.

But, going ahead. Outer space has been proven. It's not a matter of "since we can't be sure, I'll go with my own beliefs", as in the legendary god argument. In this case, the existence of outer space has been proven. If you want to argue that it doesn't exist, go on and gather evidence to disprove the huuuuuuuuuuge rolls of evidence in favour of its existence. Otherwise, it's "there is all this evidence" against "I think that". I know what I'll be going with.

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He sleeps alone
He needs no army where he's headed
'Cause he knows
That they're just ghosts
And they can't hurt him
If he can't see them, oh